A mirrorless interchangeable lens camera is a digital camera with an interchangeable lens. A mirrorless camera uses an image sensor to provide an image to the electronic viewfinder (EVF). It is called mirrorless since it does not have a movable mirror in the optical path.
Compared to single-lens reflex cameras (SLR), mirrorless cameras can be made simpler, smaller and lighter because they do not have an optical viewfinder. Such a viewfinder is composed of a mirrorhousing, a movable mirror, or a viewing prism with reticle. Neither do they need a secondary autofocus mirror, an autofocus sensor array and a separate light metering sensor.
Since mobile phones with cameras, compact cameras, superzoom cameras today all are mirrorless cameras, it is essentially only the DSLR cameras, with emphasis on the R (Reflex) which are cameras with movable mirrors.
Mirrorless cameras have until recently had two challenges keeping them from competing with top of the line DSLRs. The initial challenge was to provide an EVF with the resolution, clarity and response of direct optical viewing. The second challenge has been that the contrast detect autofocus (CDAF) initially used in mirrorless cameras requires about twice the time to acquire focus compared to that of phase detect autofocus (PDAF). Professional photographers covering sports and news events have therefore been among the last to embrace the mirrorless cameras. The latest generation mirrorless cameras, however, have PDAF pixels built into the image sensor offering fully competitive and accurate autofocus and many times faster continuous shooting with continuous autofocus than DSLRs.
With high quality images being available even from smaller sensors a new distinction other than camera price is available to photographers. As a rule, to professionally photograph studio objects, landscapes or architecture requiring rich and realistic images, a camera with a full frame or a medium format sensor is preferred. Lenses required and used for this are typically in the 20 to 200 mm focal range. With the same requirement for image quality, sports and wildlife photography then requires focal lengths from 600 to 800 mm or more. Mirrorless system cameras with smaller, high resolution sensors can here offer the advantage of the same final image coverage using shorter focal length, lighter lenses because of the so-called "crop factor" of the smaller sensor. This of course requires that the smaller sensor has a high enough pixel density to offer a final resolution equal to or better than what can be cropped out of the image from a larger sensor using the same, shorter focal length lens.
Whereas image stabilizing techniques (IS), also called vibration reduction (VR), have been available in long focal length lenses for DSLRs for some time, recent mirrorless cameras are offering in body image stabilization (IBIS) where the image sensor moves inside the camera to keep the image steady on the image sensor, counteracting any undesirable vibration or shaking of the camera during handheld exposures. The manufacturer Olympus combines this with vibration reduction in the lens to achieve five axes of total vibration reduction including two axes of rotation. The result is claimed to enable sharp and shake free handheld exposures as long as several seconds.
Electronic first and second curtain shutters for a totally quiet, electronic exposure is also becoming common. With sensor scan times down to 1/60 of a second, totally quiet electronic shutters are now useful even during short exposures of subjects in some motion without distorting the image.
The ability of a camera with autofocus to re-focus between rapid shots in a series of continuous exposures (CAF) has up until recently been possible only with high end DSLRs, and even then, because of the limited speed of the mirror mechanism, the continuous shooting speed has been limited to around half a dozen shots per second. Although autofocus speed has been the Achilles heel of the mirrorless camera, several manufacturers today boost continuous shooting with continuous refocusing between exposures at rates up to 18 frames per second with silent, electronic shutter and up to 10 frames per second with mechanical shutter.
In camera automation of high dynamic range photography (HDR) is being offered. The camera takes a series of shots under varying exposures and combines them in the camera into one high dynamic range image.
Though the resolution of any digital image sensor is limited, the IBIS mechanism in the camera allows the camera to move the image sensor in steps smaller than the image sensor pixel size to produce a multi exposure image with a resolution higher than the image sensor itself. Both high dynamic range (HDR) and high resolution images are best captured while the camera is mounted on a tripod and triggered remotely.
Many mirrorless cameras are also capable of producing an image with seemingly infinite depth of focus. The technique involves several exposures executed at various focal planes and combined in the camera to an image with seemingly infinite depth of focus.
Touch screen technology has finally been adopted by the camera industry. It is now easier to maneuver within menus and between commands thanks to intuitive touch screen technology being applied to mirrorless camera LCD displays.
Because of advances made in digital image sensor technology and electronic viewfinders, electronics are replacing most of the mechanics that once were necessary in film SLRs for framing an image through either an optical rangefinder or an optical viewfinder based on the single lens reflex mirror concept. It is likely that this evolution will continue offering even smaller and more capable mirrorless cameras in the future.
One of the pioneers in the field has been Sony Corporation who supplies a large number of other camera manufacturers with image sensors. Sony also sells cameras of their own, especially to show off advancements in their sensor and processing technology, often releasing improved designs at a rapid rate while simultaneously carefully limiting how they can and want to compete with their sensor customers.
Both Canon and Nikon, the two biggest camera manufacturers, have been struggling financially with the loss of sales of simpler and less expensive cameras to the manufacturers of camera enabled mobile phones. As a result, both Canon and Nikon have been slow in picking up on the trend toward mirrorless system cameras. However, once the market shows them where it is going we are likely to see mirrorless system cameras from both Canon and Nikon as well.
Other early players in the mirrorless system camera market are shown in the Systems Comparison below.
Whereas mobile phones with cameras, point and shoot cameras, compact cameras, superzoom cameras, compact mirrorless system cameras and mirrorless system cameras today all exist in the marketplace a trend toward elimination of the less successful categories is noticeable. While mobile phones with cameras have taken over the lion's share of the point and shoot and compact camera market, more advanced mirrorless cameras with a non-interchangeable prime lens are still being marketed. However, even more useful and common are today mirrorless cameras with a non-interchangeable zoom lens, whereas the big category is likely to become mirrorless system cameras with interchangeable lenses. An interesting trend toward the difference between the categories compact mirrorless system cameras and professional mirrorless system cameras is that while casual users may prefer a pocket size camera, sometimes with a collapsible or pancake lens, professional users demand a more substantial camera grip with good balance and comfortable ergonomics plus a battery for all day use, making new, professional mirrorless system cameras slightly larger than their compact siblings.
In 2013 Mirrorless system cameras constituted about five percent of total camera shipments. In 2015, they accounted for 26 percent of system camera sales outside the Americas, and 16 percent in the U.S.
2004-2008. The category started with Epson R-D1 (released in 2004), followed by Leica M8 (released September 2006, which isn't actually a "mirrorless" but a rangefinder camera, a system of focussing dating back to 1933 and the release of the Leica III, itself a development of the 1932 Leica II) and then the Micro Four Thirds system, whose first camera was the Panasonic Lumix DMC-G1, released in Japan in October 2008.
2009-2010. A more radical design is the Ricoh GXR (November 2009), which features, not interchangeable lenses, but interchangeable lens units – a sealed unit of a lens and sensor. This design is comparable but distinct to MILCs, and has so far received mixed reviews, primarily due to cost; As of 2012 the design has not been copied.
Following the introduction of the Micro Four Thirds, several other cameras were released in the system by Panasonic and Olympus, with the Olympus PEN E-P1 (announced June 2009) being the first in a compact size (pocketable with a small lens). The Samsung NX10 (announced January 2010) was the first camera in this class not using the Micro Four Thirds system – rather a new, proprietary lens mount (Samsung NX-mount). The Sony Alpha NEX-3 and NEX-5 (announced May 14, 2010, for release July 2010) saw the entry of Sony into the market, again with a new, proprietary lens mount (the Sony E-mount), though with LA-EA1 and LA-EA2 adapters for the legacy Minolta A-mount.
2011. In June 2011, Pentax announced the 'Q' mirrorless interchangeable lens camera and the 'Q-mount' lens system. The original Q series featured a smaller 1/2.3 inch 12.4 megapixel CMOS sensor. The Q7, introduced in 2013, has a slightly larger 1/1.7 inch CMOS sensor with the same megapixel count.
In September 2011, Nikon announced their Nikon 1 system which consists of the Nikon 1 J1 and Nikon 1 V1 cameras and lenses. The V1 features an electronic viewfinder.
2012. The Fujifilm X-Pro1, announced in January 2012, was the first non-rangefinder mirrorless with a built-in optical viewfinder. Its hybrid viewfinder overlays electronic information, including shifting framelines to compensate for parallax. Its 2016 successor, the X-Pro2, features an updated version of this viewfinder.
Beyond the interest to consumers, mirrorless has created significant interest in camera manufacturers, having potential to be an alternative in the high-end camera market. Significantly, mirrorless has fewer moving parts than DSLRs, and are more electronic, which plays to the strengths of electronic manufacturers (such as Panasonic, Samsung and Sony), while undermining the advantage that existing camera makers have in precision mechanical engineering. Sony's entry level full frame mirrorless α7 II camera has a 24MP 5 axis stabilised sensor yet is more compact and lower in cost than any full frame sensor DSLR.
Nikon announced the Nikon 1 series with a 1" sensor on September 21, 2011. It was a high-speed mirrorless which according to Nikon featured world's fastest autofocus and world's fastest continuous shooting speed (60 fps) among all cameras with interchangeable lenses including DSLRs. Canon was the last of the major makers of DSLRs, announcing the Canon EOS M in 2012 with APS-C sensor and 18 mm registration distance similar to the one used by NEX.
In a longer-term Olympus decided that mirrorless may replace DSLRs entirely in some categories with Olympus America's DSLR product manager speculating that by 2012, Olympus DSLRs (the Olympus E system) may be mirrorless, though still using the Four Thirds System (not Micro Four Thirds).
Panasonic UK's Lumix G product manager John Mitchell while speaking to the Press at the 2011 "Focus on Imaging" show in Birmingham, reported that Panasonic "G" camera market share was almost doubling each year, and that UK Panasonic "G" captured over 11% of all interchangeable camera sales in the UK in 2010, and that UK "CSC" sales made up 23% of the Interchaneable lens market in the UK, and 40% in Japan.
As of May 2010, interchangeable-lens camera pricing is comparable to and somewhat higher than entry-level DSLRs, at US$550 to $800, and significantly higher than high-end compact cameras. As of May 2011, interchangeable-lens camera pricing for entry mirrorless appears to be lower than entry-level DSLRs in some markets e.g. the U.S.
Sony announced 2011 sales statistics in September 2012, which showed that mirrorless had 50% of the interchangeable lens market in Japan, 18% in Europe, and 23% worldwide. Since that time Nikon has entered the mirrorless market, amongst other new entries.
2013. In a down-trend world camera market, mirrorless also suffered, but not much and can be compensated with increase by about 12 percent of 2013 sales in popular mirrorless domestic (Japan) market. However, mirrorless has taken longer catch on in Europe and North America—according to Japanese photo industry sources, mirrorless made up only 11.2% of interchangeable-lens cameras shipped to Europe in the first nine months of 2013, and 10.5% in the U.S. in the same period. Also, an industry researcher determined that Mirrorless sales in the U.S. fell by about 20% in the three weeks leading up to December 14, 2013—which included the key Black Friday shopping week; in the same period, DSLR sales went up 1%. In 2015, mirrorless is gaining market share in North America, while DSLR is falling, showing 16.5% $ value growth rates for mirrorless while DSLR is falling by 17% in $ value sales. In Japan, mirrorless at times outsells DSLR. Despite lowering DSLR prices 2015 sales figures due in January 2015 will show further increases of mirrorless compared to DLSRs in the ILC market. In 2015, mirrorless-cameras accounted for 26 percent of interchangeable-lens camera sales outside the Americas, although a lesser share of 16 percent in the U.S., but still  a huge increase in interchangeable lens camera market share in only two years.
2015. 2015 statistics show that overall camera sales have fallen to one third of those of 2010, due to compact cameras being substituted by camera capable mobile phones. This means that overall share of camera sales is seeing ILC market share increasing, with world volumes showing ILC having 30% for overall camera sales, of which DSLR had 77% and mirrorless 23%. In the Americas in 2015, DSLR annual sales in dollars are now falling by 16% per annum, while mirrorless sales over the same 12-month period have increased by 17%. Hence the Mirrorless market share of interchangeable lens cameras has more than doubled in two years.
2016. Late 2016 Olympus corporation announced the introduction of their OM-D E-M1 Mark II, a successor to the earlier and successful Mark I. The Mark II model retains a micro 4/3 image sensor of 17.3x13 mm featuring 20.4 MP resolution and represents a new generation of mirrorless cameras competitive with and in many respects superior to DSLR cameras. It is likely that this development will be continued by other camera manufacturers into larger APS-C, full format and medium format mirrorless system cameras.